In my effort to map out a control strategy for reactor pressure control I have been studying ionization gauges. Depending on cathode emission and plate voltage they are very linear. Given their construction and plate voltage (in the 200 to 350 volt range) their response speed should be at least in the low MHz range.
The limiting factor is going to be the current amplifier (because current is proportional to gas density). At low currents - current amplifiers tend to have low bandwidths. Should that be a problem in controlling the reactor the gauge operating conditions could be optimized for the desired control range. A second gauge operated normally could be used to turn off the gauge if pressures got too far out of range. That should give us a readout bandwidth in the 100s of KHz.
With a valve bandwidth of 1,000 Hz and a reactor fill time of 1 second with a full open control valve, pressure control to within 1% ought to be easy. A little work could bring that in to .5%. Dr. B. said that his suggested pressure was 1E-7 torr because his control system was lousy. He said the real number was about 30X higher. If that is the case an increase of density with better control should allow at least a 10X increase in pressure. For D-D that means a reactivity increase of n2/2 - about 50X. If we could keep things tight enough to get the whole 30X increase in pressure it would give a gain improvement of 450. Tight pressure control is one of the critical keys to making this work.
You can get a nice first pass of ionization gauge theory and practice from this page. Click on Introduction to Bayard-Alpert Gauges. It will bring up a [pdf] "open or save" block.
Which has got me to thinking. If POPS causes density waves in the plasma and those density waves get coupled to the neutrals it may be possible to use an ionization gauge to measure POPS frequencies. You would use a diplexer. The low frequency signals would go to the pressure controller and the high frequency signals would feed the POPS frequency controller phase detector (after suitable division if required).
BTW if you are going to use CMOS I used to highly recommend the Philips 74HC4046 however Philips is no longer in the Semiconductor business. Instead that business is now called NXP. So here is NXP version of the 74HC/HCT4046A[pdf]